Why is my Wacom Cintiq Companion eraser not erasing in sculpt mode?
Welcome to BSE Nikolai. As far as I know there is no erase in sculpt mode - Add pulls the vertex along the normal outward, and Subtract pushes them in. You can either do this by the menu buttons or holding down  CTRL  to reverse the operation. The eraser still works but has no function on the meshes. If you had for example grease pencil strokes, you could erase those even though you are in Sculpt mode. The same is also true of Edit mode.